Question

A Permanent Magnet DC motor is known to have a back-emf constant of 0.05 V/rpm. It is operating at a voltage of 90 V and has an armature resistance of 0.5 Ω. Assume that there are no rotational losses. Find: a) the starting current b) the starting torque c) the no-load speed

Answer 1

To find the starting current, use Ohm's law to calculate the current by dividing the voltage by the resistance. To find the starting torque, multiply the torque constant by the starting current. To find the no-load speed, divide the voltage by the back-emf constant.

Step-by-step explanation:

To find the starting current of a Permanent Magnet DC motor, we can use Ohm's law, which states that V = IR, where V is the voltage, I is the current, and R is the resistance. In this case, the voltage is 90 V and the resistance is 0.5 Ω. Therefore, I = V/R = 90/0.5 = 180 A.

To find the starting torque, we can use the equation T = Kt * I, where T is the torque, Kt is the torque constant, and I is the current. The back-emf constant (Kt) is given as 0.05 V/rpm. Therefore, T = 0.05 * 180 = 9 Nm.

The no-load speed can be found using the equation V = Kt * ω, where V is the voltage, Kt is the back-emf constant, and ω is the angular velocity. Rearranging the equation, we have ω = V/Kt = 90/0.05 = 1800 rpm.